To protect regional and global environment, exhaust-gas-cleaning catalytic converters and particulate-matter-capturing filters comprising ceramic honeycomb structures are used to reduce harmful substance contained in exhaust gases discharged from engines of automobiles, etc.
As shown in FIG. 2, a conventional ceramic honeycomb structure 20 comprises large numbers of flow paths 24 formed by perpendicularly crossing cell walls 23 and an outer peripheral wall 21, and its cross section shape perpendicular to the flow paths is usually substantially circular or elliptical. The outer peripheral wall 21 of the ceramic honeycomb structure 20 is held by a grip member (not shown) formed by a metal mesh, a ceramic mat, etc. in a metal container (not shown) to prevent the ceramic honeycomb structure 20 from moving during driving.
The ceramic honeycomb structure 20 is produced by the steps of (1) mixing and blending starting materials comprising a ceramic material such as cordierite powder, a molding aid, a pore-forming material, etc. with water to prepare a moldable ceramic material, (2) extruding the moldable ceramic material through a honeycomb-shaped die to form a green ceramic honeycomb body having a honeycomb structure integrally comprising an outer peripheral wall 21 and cell walls 23, and (3) drying and sintering the green body. Such steps produce a ceramic honeycomb structure 20 having predetermined shape and strength with cell walls 23 having fine pores.
Filters for cleaning exhaust gases discharged from diesel engines may use large ceramic honeycomb structures 20 having outer diameters D of 150 mm or more and length L of 150 mm or more, with cell walls 23 as thin as 0.2 mm or less, as shown in FIG. 2. In the production of such a large ceramic honeycomb structure 20 with thin cell walls, a green ceramic honeycomb body obtained from the moldable ceramic material by extrusion has such insufficient strength that it may be deformed in an outer peripheral wall 21 by its own weight, resulting in crushed cell walls 23. The sintering of a deformed green body would not provide a ceramic honeycomb structure 20 having desired strength.
To solve this problem, as shown in FIGS. 1(a) and 1(b), JP 5-269388 A discloses a honeycomb structure 10 comprising an outer peripheral wall 12 bonded to a ceramic honeycomb body 11, the outer peripheral wall 12 being formed by filling grooves 15 of cells 14a on an outer peripheral surface among large numbers of cells 14 defined by cell walls 13 with a coating material 12c comprising cordierite particles and/or ceramic fibers and colloidal oxide (colloidal silica, colloidal alumina, etc.) as main components, and drying or sintering it. JP 5-269388 A describes in Examples that the applied coating material 12c was left to stand for 24 hours in the air, and then dried at 90° C. for 2 hours to form the outer peripheral wall 12.
However, a drying method by heating from outside as described in JP 5-269388 A heats a surface portion 12s of the coating material 12c first, heat being gradually conveyed to the inside 12n. Accordingly, the surface portion 12s of the coating material 12c is first dried, and water then moves from the inside 12n to the surface and evaporates from the already dried surface portion 12s, so that the inside 12n is then dried. Accordingly, there is difference in a water content between the surface portion 12s and inside 12n of the coating material 12c during drying, and the resultant drying shrinkage difference is likely to generate cracks 16 on the coating material surface 12s. Particularly when the outer peripheral wall is thick, or when the heating temperature is high to shorten the drying time, there is larger temperature difference, resulting in more likelihood of generating cracks 16.
Like JP 5-269388 A, JP 2006-298745 A discloses a ceramic honeycomb structure 10 coated with a coating material 12c on a peripheral surface shown in FIG. 1. This reference describes that the coating material 12c is formed by a slurry comprising crushed porcelain having a particle size of 15-75 μm with a water content of 26-34% by mass, and resistant to cracking even by forced drying (drying with far-infrared rays and/or hot air), resulting in reduced production time.
However, particularly when a large ceramic honeycomb structure having an outer diameter D of 150 mm or more and a length L of 150 mm or more used for diesel engines, etc. is produced, forced drying using far-infrared rays and/or hot air would be difficult to uniformly dry the outer peripheral wall 12 even though a coating material slurry comprising the crushed porcelain described in JP 2006-298745 A were used, resulting in partially uneven drying. As a result, the outer peripheral wall 12 is provided with densified portions and less densified portions, making it likely that the low-strength, outer peripheral wall is cracked by small shock during handling.